1. Field of the Invention
The present invention relates to techniques of improving, over a wide range of load variations from light loads to heavy loads, the controllability of switching power-supply apparatuses that include a half-bridge or full-bridge system suitable for use in high-power applications in a primary-side driving circuit without there being substantially any periods during which energy is not transferred between the primary and secondary sides of the transformer.
2. Description of the Related Art
Known examples of switching power-supply apparatuses for high-power applications have included half-bridge and full-bridge converters.
A half-bridge converter is illustrated in FIG. 1. A primary-side power converter circuit includes a series circuit including a first switch circuit S1, which includes a first switching element Q1, a first capacitor C1, and a first diode D1 connected in parallel with one another, and a second switch circuit S2, which includes a second switching element Q2, a second capacitor C2 and a second diode D2 connected in parallel with one another, and a series circuit including a high-voltage-side capacitor C10 and a low-voltage-side capacitor C11, that are connected in parallel with an input power supply Vin. Furthermore, the two ends of a first primary coil Np1 of a transformer T are respectively connected to a connection point between the first switch circuit S1 and the second switch circuit S2 and to a connection point between the high-voltage-side capacitor C10 and the low-voltage-side capacitor C11.
Furthermore, in a secondary-side power converter circuit of the half-bridge converter, the anode of a third diode D3 is connected to an end of a first secondary coil Ns1 of the transformer T, the anode of a fourth diode D4 is connected to an end of a second secondary coil Ns2, the cathode of the third diode D3 and the cathode of the fourth diode D4 are commonly connected to an end of a first inductor L1 and the other end of the inductor L1 is connected to an end of a smoothing capacitor C8 and an end of a load Ro. In addition, the other end of the first secondary coil Ns1 and the other end of the second secondary coil Ns2 are commonly connected to the other end of the smoothing capacitor C8 and the other end of the load Ro.
In such a half-bridge converter, the first switching element Q1 and the second switching element Q2 are driven in a complementary manner such that the duty ratios of the first switching element Q1 and the second switching element Q2 are set so as to be approximately 50% when the input voltage and the output voltage are in a rated condition. Since the first switching element Q1 and the second switching element Q2 are short-circuited if they are both simultaneously in an on state, the elements are complementarily driven before and after a period in which both the elements are in an off state (dead time) and, therefore, in practice, the duty ratios are not exactly 50%. For example, the following two control methods exist for cases where the input voltage rises or the output voltage rises, as described below.
In a first method, control is performed such that the output voltage is reduced by complementarily driving the first switching element Q1 and the second switching element Q2 in a completely symmetrical manner such that the duty ratios of both of the first switching element Q1 and the second switching element Q2 is short and the dead time accordingly is long.
In a second method, control is performed such that the output voltage is reduced by alternately turning on the first switching element Q1 and the second switching element Q2 before and after a predetermined dead time such that the duty ratio of the first switching element Q1 is short and, conversely, the duty ratio of the first switching element Q2 is long.
However, there is a drawback with first method in that the period of time during which the first switching element Q1 and the second switching element Q2 are both off (dead time) changes each time, such that the switching elements cannot be subjected to zero voltage switching (ZVS) driving. Therefore, the second method has been preferred in recent years for switching power-supply apparatuses which require high efficiency.
However, with the second method, there is a problem in that the first switching element Q1 and the second switching element Q2 are alternately turned on before and after a predetermined dead time and, therefore, although zero voltage switching (ZVS) driving is possible, as illustrated in FIG. 5, when the load state is a light load, there is a period of time in which the current flowing through the first inductor L1 becomes 0 A and a control characteristic in which when the duty ratio of the first switching element Q1 drops, the output voltage drops, is reversed and control becomes impossible.
In addition, as described in, for example, Japanese Unexamined Patent Application Publication No. 11-262263, if a boost half bridge (BHB) system is used, although zero voltage switching is achieved and the control characteristics at times of light load can be maintained, there is a problem in that the withstand voltage of a switching element must be approximately twice the input voltage, the on resistance of a switching element having such a high withstand voltage is high, and the switching loss is large.
For example, with switching power-supply apparatuses used in general domestic electronic appliances, such as televisions, a reduction of the standby-time power is often highly desirable and stable control characteristics over a considerably wide range of load variation from a light load state in which there is approximately no load to a heavy load state are required. However, it has not been possible to produce switching power-supply apparatuses that are capable of low loss and a wide load variation range with any of the existing methods.